This invention generally relates to a system for measuring the modulation transfer function of optical systems such as image intensifier tubes and, more particularly, to improved means for normalizing such apparatus either manually or automatically.
In recent years, the image-forming quality of an optical device such as a lens or an image intensifier tube has been quantitatively by determining its light transmission efficiency at a selected spatial frequency. This newer method reduces the imprecision of the more conventional, subjective evaluation of having a technician observe the degree of resolution of an image formed by the optical device being examined. The quantitative measurement is typically referred to as the "modulation transfer function" (MTF) and it is expressed as a percentage at a given spatial frequency. For example, if a lens has an MTF of 75 percent at a spatial frequency of 100 line pairs per millimeter, that measurement means that 75 percent of the image information was passed by the lens and 25 percent of the image information was lost at that spatial frequency. Similarly, an image intensifier tube having an MTF of 85 percent at 7.5 line pairs per millimeter transmits 85 percent of the image information at that frequency and loses 15 percent.
Various devices have been proposed for attempting to measure modulation transfer function but they have encountered difficulties with speed, reliability, and accuracy. One particular shortcoming of conventional MTF measuring apparatus has been the inability to conveniently establish an absolute 100 percent MTF reference level so that, in measuring a series of similarly constructed lenses, for example, a meaningful set of MTF measurements could be obtained for the entire batch of lenses. For instance, calibration of the system disclosed in U.S. Pat. No. 3,743,427 to Weiser is accomplished by using an optical chopper and is limited, in a practical sense, to normalization only during periods when measurements are not being taken. Thus, such factors as variations in the level of background illumination and the stability of the electrical components involved during the time that the device is being measured may vary the measured value of the modulation transfer function. On the other hand, the device of U.S. Pat. No. 3,438,713 to Heynacher et al. uses the lowest spatial frequency in a series of modulated frequencies as a relative point and hence is unable to provide an absolute calibration of modulation transfer function. U.S. Pat. No. 3,489,497 to Bigelmaier uses the DC component of the photomultiplier output signal for selected spatial frequencies to regulate the sensitivity of the system. Not only is the DC system subject to bias drifts and generally more complicated than an AC system, but it does not provide an absolute MTF measurement. A somewhat cumbersome compensation system is provided by U.S. Pat. No. 3,447,874 to Back which uses an open-loop compensation system for the normalization of gain, including a number of potentiometers which are switched into and out of the circuit by a mechanical switch attached to the frequency selector (a zoom lens).
It is therfore a general object of this invention to provide a new and improved system for measuring the modulation transfer function of an optical device.
It is a further object of the present invention to provide such a system which has means for normalizing the measurements to obtain absolute MTF values at one or more spatial frequencies.
It is another object of the invention to provide such normalization automatically and continuously throughout the duration of the evaluation of the optical device under test.